How Unexpected Was the 2021 Pacific Northwest Heatwave?

Abstract

The 2021 Pacific Northwest heatwave featured record‐smashing high temperatures, raising questions about whether extremes are changing faster than the mean, and challenging our ability to estimate the probability of the event. Here, we identify and draw on the strong relationship between the climatological higher‐order statistics of temperature (skewness and kurtosis) and the magnitude of extreme events to quantify the likelihood of comparable events using a large climate model ensemble (Community Earth System Model version 2 Large Ensemble [CESM2‐LE]). In general, CESM2 can simulate temperature anomalies as extreme as those observed in 2021, but they are rare: temperature anomalies that exceed 4.5σ occur with an approximate frequency of one in a hundred thousand years. The historical data does not indicate that the upper tail of temperature is warming faster than the mean; however, future projections for locations with similar climatological moments to the Pacific Northwest do show significant positive trends in the probability of the most extreme events.

Sixth Oregon climate assessment

“…A recent study estimated that this heat wave was about two degrees Fahrenheit warmer than it would have been without human influence on the climate (Bercos-Hickey et al 2022), consistent with the increase in mean temperature. An event of this magnitude is likely to occur once in 1000 to 100,000 years (McKinnon andSimpson 2022, Thompson et al 2022). A heat dome would have occurred without climate change, but maximum temperatures would not have been as high (McKinnon and Simpson 2022).…”

“…An event of this magnitude is likely to occur once in 1000 to 100,000 years (McKinnon andSimpson 2022, Thompson et al 2022). A heat dome would have occurred without climate change, but maximum temperatures would not have been as high (McKinnon and Simpson 2022). estimated that a heat wave of similar magnitude will recur about once in six years by the end of the twenty-first century if concentrations of greenhouse gases do not decrease.…”

Anthropogenic Contributions to the 2021 Pacific Northwest Heatwave

“…It is clear that ensemble sizes in the standard CMIP database are not large enough to construct such distributions and provide robust attribution statements. However, recent developments of larger climate model ensemble simulations (up to 40 individual realizations) do provide some insight as to the rarity of such events (McKinnon & Simpson, 2022). Despite these limitations of conventional attribution methods, it is highly likely that anthropogenic climate change influenced the 2021 PNW heatwave.…”

North-West Europe hottest summer temperatures rising much faster than the mean

“…Station data for many parts of Europe show an increase in day-to-day temperature variability within the summer season since the 1960s (Krauskopf and Huth, 2022), while increased temperature variability is necessary to account for the magnitude of the 2003 European heat wave (Schär et al, 2004). On the other hand, McKinnon and Simpson (2022) and Thompson et al (2022) found no evidence for faster warming of extremes, relative to the mean for the Pacific North-West.…”